Pressure-independent control valves, which are commonly referred to by the acronym “PICV,” are a type of control valve for controlling the flow rate of a fluid passing through a conduit. PICVs typically use two fluid flow control elements disposed in series. The first control element can control a pressure difference across the second control element, which controls the flow of fluid at the controlled pressure difference. In conventional applications, a hydraulically operated differential pressure control element is used in conjunction with an electrically actuated flow control valve element. The control valve element can respond to a command signal from a controller, while the hydraulically operated differential pressure control element adjusts the pressure difference across the control valve element.
For stabilizing the differential pressure across the control valve element, the hydraulically operated differential pressure control element is configured to adjust its flow resistance dynamically in response to pressure fluctuations in the system. In this way, the pressure difference is dynamically maintained constant to offset changes in flow, which are driven by system pressure fluctuations. The result of the controlled pressure difference across the control valve element means that a predictable and repeatable flow through the control valve element will correspond to each valve position of the control valve element.
PICVs are widely used in various fluid systems such as closed circuit high flow networks used, for example, in district cooling and heating networks. Advantages of PICV systems include high flow accuracy that is independent of system pressure fluctuations. High flow accuracy, in turn, permits satisfactory network loop fluid balancing and control stability. However, PICV systems also have disadvantages. For example, by using two control elements, a higher pressure drop across the system is inherently induced, which tends to increase pumping pressure for compensating the higher pressure loss. Moreover, the response time for PICV systems can be sluggish, especially due to hydro-mechanical controlling elements used to control the hydraulic pressure difference control element. Further, the operating range of the system in terms of flow and pressure fluctuation capability can be limited.